1. Field of the Invention
The present invention relates to free wheels. More specifically, the present invention relates to disengageable pulleys fitted with free wheels used for example in the drive pulleys of motor vehicle alternators.
2. Description of the Relevant Art
Disengageable pulleys fitted with free wheels are known per se and are coming into increasing use to solve the harmful effects of acyclical phenomena or sudden decelerations of the engine which occur in internal-combustion engines, especially at low engine speeds and more especially in diesel engines, the danger being that a drive belt connected to the engine via a crankshaft pulley may decelerate suddenly while a driven pulley, for example an alternator pulley, will tend, by inertia, to continue to rotate at the same speed.
If there is a rigid coupling between the crankshaft pulley and the alternator shaft, the belt will come under severe stresses when these instantaneous changes of speed take place.
Such changes of speed produce harmful effects such as abnormal fatigue of the belt, which may break, or slide over the pulley, or the portions of belt between the pulleys may vibrate.
To attenuate these phenomena, a free wheel has been incorporated between the driven pulley and the driven shaft to provide for a temporary decoupling of the pulley from the shaft in the event of sudden deceleration of said pulley.
Such a disengageable pulley is known particularly from document JP 2005-282856 and generally includes two rolling-contact bearings located on either side of the pulley and, between the two rolling-contact bearings, a free wheel provided with a plurality of wedge-type cams and a cage containing a plurality of windows to house said cams.
In order to limit the wear on these cams, that document has the holding cage fixed to the driven shaft in order to drive the cage and the shaft at the same angular velocity. In this way, when the pulley is in the disengaged position in which it operates as a free wheel, slip occurs only between the cams and a raceway, located inside the pulley, which is correctly lubricated by the centrifugal force tending to throw the lubricant out towards said raceway.
However, with such a pulley, the cage has no degree of angular freedom relative to the driven shaft. The applicant has observed that this can be detrimental to the reliability of operation of the free wheel owing in particular to the fact that it creates a risk of interference between the cams and the edges of the windows of the holding cage at the moment when the cams tilt over.
Also known, this time from document FR-A-2 856 759, is a disengageable pulley provided with two holding cages for the coaxial wedge cams in which the inner cage includes friction elements that come into contact with the driven shaft. The outer cage includes friction elements in the form of clips in contact with the pulley.
In a disengaged position or when free-wheeling, the frictional torque between the friction elements and the pulley tends to retard the inner cage relative to the outer cage. The edges of the windows formed in the inner cage thus come into contact with the wedge cams and tilt them over. The cams are thus no longer in contact with the driven shaft.
The problem with such a device is its use of two holding cages, one provided with friction elements in the form of clips, which increases the cost price of the device.
In addition, American U.S. Pat. No. 4,018,318 discloses a disengageable pulley provided with a wedge cam cage, a friction element in contact with the driven shaft, and a drive rod attached to the cage to turn the shaft through the friction element. The friction element is an open ring with radial elasticity to allow it to be fitted into a groove formed on the inner transmission element.
That pulley has the drawback that it uses two separate elements to turn the driven shaft with the cage. Moreover, given the frictional contact between the open ring and the driven shaft, under certain conditions of operation of the pulley the cage has no degree of angular freedom with respect to the driven shaft. The applicant has found that this can be detrimental to the pulley's reliability of operation.